The present invention relates to a safety device to make a closed battery explosion-proof and a closed battery with the safety device.
Recently, a lithium battery with non-water electrolytic solution and a secondary battery with non-water electrolytic solution for a lithium ion battery have been broadly employed for pocket electronic tools.
Although the secondary battery has a high electro motive force, the secondary battery would explode in case of increasing inner pressure caused by chemical reaction in an electrode member with a positive electrode and a negative electrode installed in an outer container. For example, when a non-aqueous electrolytic solution battery such as a lithium secondary battery is in an overcharge condition or a large amount of electric current is flown into the battery in a short circuit condition caused by a wrong operation, non-aqueous electrolytic solution is absorbed in the electrode member and gas is produced. When the gas is continuously produced in the outer container and the inner pressure of the outer container is increased, the battery would eventually explode.
In order to avoid such an explosion, a closed battery for pocket electronic tools was developed as shown in Japanese Patent Laid-Open Publication No. 6-338305.
In the conventional art, a positive electrode lid provided at an edge of an outer container comprises a metallic porous plate affected as the inner most lid and connected to a positive electrode of an electrode member through a positive electrode lead, a metallic explosion-proof valve affected as an intermediate lid electrically connected to the metallic porous plate through a central adhered portion and a metallic cap terminal electrically connected to the metallic explosion-proof valve.
Under the structure as described above, when the inner pressure of the battery is increased, the metallic porous plate and the metallic explosion-proof plate are electrically isolated by breaking the central adhered portion and gas produced in the inside of the battery is discharged by breaking a part of the metallic explosion-proof valve so as to avoid an explosion of the battery.
However, in a conventional closed battery, some drawbacks remain. Although a central adhered portion is formed by welding a central portion of a metallic porous plate and a metallic explosion-proof valve by spot welding, it is difficult to weld all the portions uniformly by spot welding and a welding strength of each closed battery is not constant. As the result, the inner pressure of a battery for isolating a communication between the metallic porous plate and the metallic explosion-proof plate is not constant. In the case of a closed battery, although the inner pressure of the battery rises to a predetermined isolation pressure, a communication between the metallic porous plate and the metallic explosion-proof can not be isolated. Thus, a reliability of a closed battery is damaged in view of safety.
In order to solve the above problems, a purpose of the present invention is to provide a safety device of a closed battery in which a pressure plate and a shield plate can be isolated certainly and the safety of the closed battery can be maintained sufficiently even if the inner pressure of the battery exceeds a predetermined level, and a closed battery with the safety device.
To accomplish the above purpose, a safety device of a closed battery as claimed in claim I comprises a positive electrode lid attached at one end of an outer container, the positive electrode lid including a pressure plate effected as the innermost lid and electrically connected to a positive electrode of an electrode member through a positive electrode lead, a shield plate effected as an intermediate lid and electrically connected to the pressure plate through a central contacting portion, and a sealing plate affected as the outermost lid and electrically connected to the shield lid wherein at least one gas flow hole is provided at the pressure plate so as to communicate the internal space of the outer container with a contacting space between the pressure plate and the shield plate. A central contacting portion includes a protrusion which protrudes from a-central portion of the pressure plate toward the shield plate and having a first flat contacting surface and a second contacting surface provided at the central portion of the shield plate and contacting with the first flat contacting surface of the protrusion. The pressure plate and the shield plate are isolated when the inner pressure level of the outer container exceeds a predetermined electric current isolation pressure.
A safety device as claimed in claim 2 comprises a positive electrode lid attached at one end of an outer container, the positive electrode lid including a pressure plate effected as the innermost lid and electrically connected to a positive electrode of an electrode member through a positive electrode lead, a shield plate effected as an intermediate lid and electrically connected to the pressure plate through a central contacting portion, and a sealing plate affected as the outermost lid and electrically connected to the shield lid, wherein at least one gas flow hole is provided at the pressure plate so as to communicate the internal portion of the outer container with a contacting space between the pressure plate and the shield plate. A plurality of circular grooves are formed at a substantially central portion of the shield plate except for connecting tab portions which are coaxially notched and alternatively arranged with an angle of 180xc2x0 so as to oppose the adjacent grooves of each other. The diameter of the grooves gradually increases from the inner side to the outer side and the pressure plate and the shield plate are isolated when the inner pressure level of the outer container exceeds a predetermined electric current isolation pressure.
A safety device as claimed in claim 3 comprises a positive electrode lid attached at one end of an outer container, the positive electrode lid including a pressure plate effected as the innermost lid and electrically connected to a positive electrode of a electrode member through a positive electrode lead, a shield plate effected as an intermediate lid and electrically connected to the pressure plate through a central contacting portion, and a sealing plate affected as the outermost lid and electrically connected to the shield lid, wherein at least one gas flow hole is provided at the pressure plate so as to communicate the internal space of said outer container with a contacting space between the pressure plate and the shield plate. The central contacting portion includes a protrusion protruded from a central portion of the pressure plate toward the shield plate and having a first flat contacting surface and a second contacting surface provided at a central portion of the shield plate and contacting with the first flat contacting surface of said protrusion, a plurality of circular grooves at a portion surrounding with the second flat contacting surface of the shield plate except for connecting tab portions which are coaxially notched and arranged alternatively with an angle of 180xc2x0 so as to oppose the adjacent grooves of each other. The diameter of said grooves gradually increases from the inner side to the outer side. A valve layer is formed at each circular groove by adhering a metallic foil piece on a the side surface of said shield plate which abuts the pressure plate when the inner pressure level of the outer container exceeds a predetermined electric isolation pressure, the second flat contacting surface of the shield plate is released from the first flat contacting surface of the pressure plate so as to isolate an electric communication between the pressure plate and the shield plate. When the pressure level of the outer container exceeds a predetermined layer break pressure, each valve layer is broken. It is preferable that the shield plate and the metallic foil piece are made of a clad metal plate.
In the safety device, it is preferable that a circular shaped PTC thermister element be provided between the shield plate and the sealing plate.
Accordingly, in a normal condition, an electric communication between the pressure plate and the shield plate in a closed space is certainly maintained by contacting the first flat contacting surface provided on the protrusion of the pressure plate and the second flat contacting surface of the shield plate. On the other hand, when the inner pressure of the battery rises rapidly and exceeds a predetermined electric current isolation pressure, the shield plate, particularly the second flat contacting surface of the shield plate is released from the first flat contacting surface by producing cracked gas so as to isolate the electric communication between the pressure plate and the shield plate. Thus, the cracked gas can be prevented from increasing and being discharged to an exterior of the battery. In such a case, the second flat contacting surface is provided at a central portion of a plurality of circular grooves coaxially and alternatively arranged with an angle of 180, so that the second flat contacting surface can be released from the first contacting surface quickly. Thus, the electric communication between the pressure plate and the shield plate can be quickly isolated once the predetermined electric current isolation pressure has been reached. In such a case, the second flat contacting surface is plastically deformed so that the second flat contacting surface avoids contact with the first flat contacting surface again. If even numbers of circular grooves are formed, the second flat contacting surface can be removed from the first flat contacting surface while both surfaces are maintained a parallel to each other.
The electric isolation as described above is effected, if chemical reaction in the outer container proceeds and cracked gas is produced and the inner pressure rises further and exceeds a predetermined layer break pressure. Cracked gas can be discharged to the exterior of the battery through at least one gas flow hole formed at the pressure plate, the contacting space, at least one circular groove (valve layer) and at least one gas discharging hole by breaking the circular groove (valve layer) formed at the shield plate.
A shield plate may be made of a clad metal plate. A valve layer may be formed by a portion of the metallic foil piece for covering with an inner circular groove and an outer circular groove. The thickness of the metal plate is preferably about 50 um and the thickness of the metallic foil piece is preferably 10 I.L m.
For example, as disclosed in Japanese Patent Laid-Open Publication No. 1-224184 filed by the present inventors, such a clad metal plate is manufactured by a glow discharge method in which a grounded metallic substrate having a contacting surface and a grounded metallic foil piece are effected as one electrode A and the other electrode B supported by an insulator are applied with alternative current I to 50 MHz in inactive gas under extreme low pressure of 1xc3x9710xe2x88x921 to 1xc3x97104 Torr. The area of the electrode A exposed with plasma caused by the glow discharge is one third less than area of the electrode B and etched by a sputter etching method.
It is preferable that a circular shaped PTC thermister element is provided between a shield plate and a sealing plate. The PTC thermister element reduces electric current while the temperature of the closed battery increases so as to avoid an explosion caused by excess current.